Under circumstances where mist/fog exists, as it is illustrated in FIG. 8, reflection light from an object to be captured is attenuated due to a scattering of particles of mist/fog while routing from the object to a camera sensor. At the same time, environment light is also scattered by the particles of mist/fog and thus scattered environment light also reaches the camera sensor. Therefore, the light applied to the camera sensor becomes mixed light of the attenuated reflection light from the object and the scattered environmental light. Observation light I(x, λ) having a wavelength λ at a pixel location x is expressed by an equation (1) by using reflection light J(x, λ) and the environmental light A(λ) at the pixel location x. Here, t(x, λ) indicates transmittance of the reflection light. The transmittance of the reflection light is expressed by an equation (2) by using a diffusion coefficient k per unit distance and a distance d to the object in a case where a condition of the environmental atmospheric air is uniform.I(x,λ)=t(x,λ)·J(x,λ)+(1−t(x,λ))·A(λ)  (1)t(x,λ)=exp(−k(λ)·d(x))  (2)
In a visible light wavelength region, the scattering of light caused by particles of mist/fog is considered as being the same independent from a wavelength of the light. Thus, the observation light I(x, λ) can be expressed by an equation (3).I(x,λ)=t(x)·J(x,λ)+(1−t(x))·A(λ)t(x)=exp(−k·d(x))  (3)
In a video image restoration technique under a condition where mist/fog exists, unattenuated reflection light J(x, λ) from the object is estimated from the observation light I(x, λ) to output an image. More specifically, by estimating a transmittance t(x) of the reflection light, the reflection light J(x, λ) is calculated using an equation (4).
                              J          ⁡                      (                          x              ,              λ                        )                          =                                            1                              t                ⁡                                  (                  x                  )                                                      ⁢                          I              ⁡                              (                                  x                  ,                  λ                                )                                              -                                                    1                -                                  t                  ⁡                                      (                    x                    )                                                                              t                ⁡                                  (                  x                  )                                                      ⁢                          A              ⁡                              (                λ                )                                                                        (        4        )            
In the above described estimation and restoration, 2 pieces of information such as the reflection light J(x, λ) and the transmittance t(x) are required to be estimated for each pixel from the observation light I(x, λ), so that the equation results in being an ill-posed problem where no solution can be found. Consequently, it is required that optimum solutions of the reflection light J(x, λ) and the transmittance t(x) should be estimated based on preliminary provided knowledge about the environment.
A certain number of techniques in which the reflection light and the transmittance are estimated for removal of mist/fog are proposed to date. Among them, a method for performing correction processing with respect to a piece of image as an input is described below with reference to non-patent document 1 and non-patent document 2.
The non-patent document 1 discloses a technique in which a restoration image is generated based on a statically-obtained knowledge that a nature description image without mist/fog generally includes, around a target pixel, a pixel whose value is 0 in either one of a red (R) channel, a green (G) channel, and a blue (B) channel. Therefore, if a pixel whose value is 0 does not exist around a target pixel, assuming that such situation is a result of an effect of superimposition of environmental light due to mist/fog, transmittance is calculated based on an amount of the superimposition.
Further, the non-patent document 2 discloses a method for separating reflection light and environmental light from each other focusing upon uncorrelation between a distance to a texture of an object and a distance to the object (i.e., a degree of superimposition of environmental light due to mist).